1. Field of the Invention
This invention relates to a lens meter capable of measuring the characteristics of a lens to be inspected while immersing the lens in a liquid, and relates to a measuring vessel for use in the lens meter.
2. Description of Related Art
A lens meter is known in which a lens to be inspected is laid on a lens receiving plate, a beam of measuring light emitted by a light source is then projected onto the lens, and the measuring light which has passed through the lens is received by a photoelectric sensor. The lens meter calculates a difference between a light receiving position on the photoelectric sensor where the measuring light is received when the lens is laid on the lens receiving plate and a light receiving position on the photoelectric sensor where the measuring light is received when the lens is not laid thereon. According to the difference, the optical characteristic of the lens, namely, the power distribution of the lens is measured in the air. This type of lens meter is used to measure the optical characteristic of a lens, such as a spectacle lens or a hard contact lens (HCL), which is rigid in material.
Some of lenses to be inspected are soft in material. If a soft contact lens (SCL), for example, is laid on the lens receiving plate to measure the optical characteristic thereof in the air, the shape of the soft contact lens is deformed during measurement because of gravity. Additionally, the soft contact lens has another feature in that its material itself holds plenty of water. For this reason, if much time is consumed for measurement, the soft contact lens is dried up with the lapse of time and thereby undergoes a change in refractive index of its material itself. This makes it difficult to accurately measure the optical characteristic of the soft contact lens by the use of the conventional lens meter in which the optical characteristic of a lens to be inspected is measured in the air. In addition, the adhesion of water droplets to the surface of the lens causes the scattering of measuring light and makes it difficult to accurately measure the optical characteristic thereof.
In order to solve these problems, there has been attempted the experiment of immersing a lens to be inspected in a liquid and measuring the optical characteristic of the lens. Referring especially to a soft contact lens, a lens meter has been developed in which the soft contact lens is immersed in a liquid (e.g., physiological salt solution) contained in a transparent vessel (e.g., transparent cell) so as to prevent the soft contact lens from being naturally dried up and being deformed by its own weight and thereafter the optical characteristic of the soft contact lens is measured. This type of lens meter is capable of measuring the optical characteristic of the soft contact lens with accuracy because the soft contact lens is prevented from being naturally dried up and being deformed by its own weight.
However, since the refractive index of a liquid is larger than that of air, the optical characteristic of the soft contact lens which has been measured in the liquid apparently becomes smaller than that measured in the air. Generally, measurement values obtained by measurement in the air are used for the optical characteristic of a lens, such as a spectacle lens or a contact lens, to be inspected. For this reason, when a contact lens is measured, a difference occurs between a measurement value obtained in the liquid and a measurement value obtained in the air. Conventionally, this difference has been corrected in such a way that the refractive index of the material of a soft contact lens and the refractive index of a liquid in which the soft contact lens is immersed are input to an arithmetic means of a lens meter, so that a measurement value obtained by measurement of the soft contact lens in the liquid is converted into a measurement value obtained by measurement thereof in the air.
In detail, on the supposition that the thickness of the contact lens is very thin, the refractive index So of the contact lens in the air is obtained by the following equation: EQU S.sub.0 =1/f.sub.0 =(n-1)(1/r.sub.1 -1/r.sub.2) (1)
wherein f.sub.0 is a focal length of the contact lens in the air, n is a refractive index of material of the contact lens, and r.sub.1 and r.sub.2 are radii of curvature of the respective surfaces of the contact lens.
On the other hand, the refractive index S.sub.w of the contact lens immersed in a liquid is obtained by the following equation: EQU S.sub.w =1/f.sub.w ={(n-n.sub.w)/n.sub.w }(1/r.sub.1 -1/r.sub.2)(2)
wherein f.sub.w is a focal length of the contact lens in the liquid, and n.sub.w is a refractive index of the liquid.
In this example, let it be supposed that a beam of measuring light passes through a transparent cell and reaches a light receiving portion of the lens meter through the air. Additionally, let it be supposed that both a liquid layer existing on the side of the rear surface of the transparent cell and a cell wall of the transparent cell are thin enough to be negligible.
If so, Eq. (2) can be approximated to the following equation: EQU S.sub.v 1/f.sub.v =1/(n.sub.w.f.sub.w) (3)
wherein S.sub.v is an apparent refractive index, and f.sub.v is an apparent focal length.
That is, the apparent refractive index S.sub.v differs from the refractive index S.sub.0 in the air.
The refractive index S.sub.0 in the air is obtained from the apparent refractive index S.sub.v in the liquid on the basis of the following equation: EQU S.sub.0 =(n-1) {S.sub.v.n.sub.w /(n-n.sub.w)} (4)
This means that the apparent refractive index S.sub.v in the liquid is convertible to the refractive index S.sub.0 in the air if both the refractive index n.sub.w of the liquid and the refractive index n of the material of the contact lens are known.
In some cases, there is a need to measure optical characteristics of soft contact lenses fitted over the eyes of a subject in an ophthalmic hospital or a contact lens clinic. In these cases, it is often impossible to trace a manufacturer (maker) of the soft contact lenses fitted over the subject's eyes or ascertain a trade mark including, for example, an article number thereof. Therefore, the refractive index of material of the soft contact lens is often unknown. Even if data concerning the manufacturer has been obtained, the refractive index of the material of the soft contact lens varies because of adhesion of protein to the soft contact lens which is caused by wearing the soft contact lens continuously. This variation is unpredictable. Therefore, it is impossible to convert a measurement value of the soft contact lens measured in a state of being immersed in a liquid into a measurement value of the identical soft contact lens measured in the air with accuracy.